Menu Healthy Lifestyle Diseases & Conditions First Aid Symptoms Tests & Procedures Videos Definitions FAQs Venous thrombosisUpdated: 2017-06-13OverviewVenous thrombosis, or thrombophlebitis, occurs when a blood clot blocks 1 or more veins, typically in the legs. It can be life-threatening if the clot gets loose and travels to the lungs. The condition can be caused by injury, surgery, or prolonged inactivity. © 1998-2022 Mayo Foundation for Medical Education and Research(MFMER). All rights reserved. Terms of Use